This application claims the benefit of priority under 35 U.S.C. §119(e) of U.S. Provisional Patent Application No. 60/849,261 filed on Oct. 3, 2006.
The present invention relates to methods of coating paperboard and of making paper with hollow core binders, and to the coated paper and paperboard articles formed by such methods. More particularly, the present invention relates to methods of coating paperboard and of making paper with an aqueous dispersion comprising binder coated void containing polymeric pigment particles in which the binder has a glass transition temperature sufficient to provide coating films, provide in-process durability, and, thereby, enable high gloss paperboard coatings and a low cost approach to making paper and coating paperboard.
To enable strong paperboard coatings of a desirable opacity and brightness, the artisan would conventionally use more binder and more titanium dioxide. However, intense competition in the paper making and paperboard coating industries limits the amount of costly binder and titanium dioxide that can be used. Accordingly, lower amounts of conventional binder and titanium dioxide are needed in a paperboard coating to realize a performance similar to existing paperboard coatings, thereby reducing coating and paper making cost for the paper(board) manufacturer.
In addition, the energy requirements of processing conventional paperboard coatings have proven costly, especially where a mid-gloss or high-gloss coating is desired. Plastic pigments such as solid polystyrene beads, hollow polymer and organic opacifier pigments, have been used to increase opacity and gloss in paper and paperboard coatings. However, such coatings in are known for some potential drawbacks such as reduced coating strength, increased coating and print mottle, and higher costs.
Binder coated hollow sphere pigments provide both binding and light scattering characteristics, thus enabling simplified formulations of coating and wet papermaking materials. For example, U.S. Pat. No. 6,139,961, to Blankenship et al., discloses aqueous dispersions of water-insoluble core/sheath polymer particles used to coat paper and paperboard. In the core/sheath particles, the core contains one or more void encapsulated by a first shell polymer having a glass transition temperature Tg greater than 50° C.; further, polymerized on the first shell is a second shell polymer having a glass transition temperature of −15° C. to −50° C. The second shell of the core/sheath polymer particles comprises at least 15 wt. % of the total weight of the first shell polymer and the second shell polymer; and the weight ratio of the core polymer to the first shell polymer is from 1:2 to 1:100. The Blankenship et al. dispersions provide opacity to coatings containing them. However, the coatings of Blankenship et al. provide inadequate strength to protect the coated article from damage during processing and do not provide an acceptably high gloss coating. As a result, one must use additional binder and opacifying pigment to achieve acceptable coating strength and gloss, which can be very expensive.
The Applicant has endeavored to solve the problem of providing a low cost, mid-gloss or high-gloss paperboard coating that can withstanding the processing of paperboard coatings.